Usually allows for more characters per line, packing more into the same amount of space (than the same text set left-aligned).

May require extra attention to word and character spacing and hyphenation to avoid unsightly rivers of white space running through the text.

May be more familiar to readers in some types of publications, such as books and newspapers.

Some people are naturally drawn to the "neatness" of text that lines up perfectly on the left and right.

About Left-Aligned Text

Often considered more informal, friendlier than justified text.

The ragged right edge adds an element of white space.

May require extra attention to hyphenation to keep right margin from being too ragged.

Generally type set left-aligned is easier to work with (i.e. requires less time, attention, and tweaking from the designer to make it look good (Sumber: http://desktoppub.about.com/cs/typelayout/a/justifyleft.htm)

Alignment
Flush left, also called "ragged right," is the most readable alignment. It provides uniform or normal letter and word spacing, minimizes awkward hyphenation of words and provides the eye with a common starting point for each line. Any significant amount of text set centered or ragged left is very difficult to read because the eye must search for the start of each new line.

Justified alignment—flush left and right—compresses or expands letter and word spacing to fit a given line and can produce awkward hyphenation of words. The disadvantages of justified text can be reduced by increasing the line length or by decreasing the point size of the type, but that also may reduce readability.

On the Web, flush left is the universal standard. (Thom Myers, Western Michigan University. Link: http://wmich.edu/writing/readability).

Best Way to Align Text on a Web
Many people with cognitive disabilities have a great deal of trouble with blocks of text that are justified (aligned to both the left and the right margins). The spaces between words create "rivers of white" running down the page, which can make the text difficult for some people to read.

This failure describes situations where this confusing text layout occurs. The best way to avoid this problem is not to create text layout that is fully justified (aligned to both the left and the right margins).

Try looking at the reviews on Amazon.com, postings to your facebook wall, tweets from your favorite celebrities, and search engine listing results from Google. That's enough to tell me that left-aligned seems to be the way to go. (http://ux.stackexchange.com/questions/3199/best-way-to-align-text-on-a-website)

Justified Alignment
Most website readers prefer to have their text set out in fairly narrow columns, and with the text aligned left. This makes it easier to read.

Justified Alignment is is actually non-standard for the web, and the W3C standards commitee state the following :

Many people with cognitive disabilities have a great deal of trouble with blocks of text that are justified (aligned to both the left and the right margins). The spaces between words create “rivers of white” running down the page, which can make the text difficult for some people to read. This failure describes situations where this confusing text layout occurs. The best way to avoid this problem is not to create text layout that is fully justified (aligned to both the left and the right margins). (http://dmjcomputerservices.com/blog/why-you-shouldnt-align-html-justify).

Left-aligned text is easier to read than right-aligned text
Full justification (where words are stretched so that they meet both the left and right margin, as in this paragraph) is only effective with pretty long lines of text (40chars+).

However, on-screen text is easier to read in narrower columns, which makes it hard to justify full justification! In my opinion, a web page is the wrong place for fully-justified copy, because it doesn’t have the resolution to implement it smoothly. (http://webdesignfromscratch.com/basics/readability/#alignment).